Human immunodeficiency virus (HIV) induces a persistent and progressive infection leading, in the vast majority of cases, to the development of the acquired immunodeficiency syndrome (AIDS) (Barre-Sinoussi et al., 1983, Science 220: 868-870; Gallo et al., 1984, Science 224:500-503). There are at least two distinct types of HIV: HIV-1 (Barre-Sinoussi et al., 1983, Science 220:868-870; Gallo et al., 1984, Science 224:500-503) and HIV-2 (Clavel et al., 1986, Science 233:343-346; Guyader et al., 1987, Nature 326:662-669). In humans, HIV replication occurs prominently in CD4.sup.+ T lymphocyte populations, and HIV infection leads to depletion of this cell type and eventually to immune incompetence, opportunistic infections, neurological dysfunctions, neoplastic growth, and ultimately death.
HIV is a member of the lentivirus family of retroviruses (Teich et al., 1984, RNA Tumor Viruses, Weiss et al., eds., CSH-press, pp. 949-956). Retroviruses are small enveloped viruses that contain a single-stranded RNA genome, and replicate via a DNA intermediate produced by a virally-encoded reverse transcriptase, an RNA-dependent DNA polymerase (Varmus, H., 1988, Science 240:1427-1439). Other retroviruses include, for example, oncogenic viruses such as human T-cell leukemia viruses (HTLV-1,-II,-III), and feline leukemia virus.
The HIV viral particle consists of a viral core, composed in part of capsid proteins designated p24 and p18, together with the viral RNA genome and those enzymes required for early replicative events. Myristylated gag protein forms an outer viral shell around the viral core, which is, in turn, surrounded by a lipid membrane envelope derived from the infected cell membrane. The HIV envelope surface glycoproteins are synthesized as a single 160 kilodalton precursor protein which is cleaved by a cellular protease during viral budding into two glycoproteins, gp41 and gp120. gp41 is a transmembrane glycoprotein and gp120 is an extracellular glycoprotein which remains non-covalently associated with gp41, possibly in a trimeric or multimeric form (Hammerskjold, M. and Rekosh, D., 1989, Biochem. Biophys. Acta 989:269-280).
HIV, like other enveloped viruses, introduces viral genetic material into the host cell through a viral-envelope mediated fusion of viral and target membranes. HIV is targeted to CD4.sup.+ cells because a CD4 cell surface protein (CD4) acts as the cellular receptor for the HIV-1 virus (Dalgleish et al., 1984, Nature 312:763-767; Klatzmann et al., 1984, Nature 312:767-768; Maddon et al., 1986, Cell 47:333-348). Viral entry into cells is dependent upon gp120 binding the cellular CD4 receptor molecules (Pal et al., 1993, Virology 194:833-837; McDougal et al., 1986, Science 231:382-385; Maddon et al., 1986, Cell 47:333-348), explaining HIV's tropism for CD4.sup.+ cells, while gp41 anchors the envelope glycoprotein complex in the viral membrane. The binding of gp120 to CD4 induces conformational changes in the viral glycoproteins, but this binding alone is insufficient to lead to infection (reviewed by Sattentau and Moore, 1993, Philos. Trans. R. Soc. London (Biol.) 342:59-66).
Studies of HIV-1 isolates have revealed a heterogeneity in their ability to infect different human cell types (reviewed by Miedema et al., 1994, Immunol. Rev. 140:35-72). The majority of extensively passaged laboratory strains of HIV-1 readily infect cultured T cell lines and primary T lymphocytes, but not primary monocytes or macrophages. These strains are termed T-tropic. T-tropic HIV-1 strains are more likely to be found in HIV-1 infected individuals during the late stages of aids (Weiss et al., 1996, Science 272:1885-1886). The majority of primary HIV-1 isolates (i.e., viruses not extensively passaged in culture) replicate efficiently in primary lymphocytes, monocytes and macrophages, but grow poorly in established T cell lines. These isolates have been termed M-tropic. The viral determinant of T- and M-tropism maps to alterations in the third variable region of gp120 (the V3 loop)(Choe et al., 1996, Cell 85:1135-1148; Cheng-Mayer et al., 1991, J. Virol. 65:6931-6941; Hwang et al., 1991, Science 253:71-74; Kim et al., 1995, J. Virol., 69:1755-1761; and O'Brien et al., 1990, Nature 348:69-73). The characterization of HIV isolates with distinct tropisms taken together with the observation that binding to the CD4 cell surface protein alone is insufficient to lead to infection, suggest that cell-type specific cofactors might be required in addition to CD4 for HIV-1 entry into the host cell.
Recently, certain chemokines produced by CD8.sup.+ T cells have been implicated in suppression of HIV infection. The chemokines RANTES (regulated on activation normal T cell expressed and secreted), macrophage-inflammatory protein-1.alpha. and -1.beta. (MIP-1.alpha. and MIP-1.beta., respectively), which are secreted by CD8.sup.+ T cells, were shown to suppress HIV-1 p24 antigen production in cells infected with HIV-1 or HIV-2 isolates in vitro (Cocchi et al., 1995, Science 270:1811-1815). Additionally, high levels of these chemokines have been found to be secreted by CD4.sup.+ T lymphocytes in individuals that have been exposed to HIV-1 on multiple occasions but, remain uninfected (Paxton et al., 1996, Nature Med. 2:412-417). While RANTES, MIP-1.alpha. and MIP-1.beta. alone or in combination, potently suppress a variety of primary HIV-1 isolates and macrophage tropic isolates, such as HIV-1.sub.BaL, some established laboratory strains, such as HIV-1.sub.IIIB, are refractory to inhibition of infection or replication by these chemokines (Cocchi et al., 1995, Science 270:1811-1815).
Chemokines, or chemoattractant cytokines, are a subgroup of immune factors that have been shown to mediate chemotactic and other pro-inflammatory phenomena (See, Schall, 1991, Cytokine 3:165-183). Chemokines are small molecules of approximately 70-80 residues in length and can generally be divided into two subgroups, .alpha. which have two N-terminal cysteines separated by a single amino acid (CxC) and .beta. which have two adjacent cysteines at the N terminus (CC). RANTES, MIP-1.alpha. and MIP-1.beta. are members of the .beta. subgroup (reviewed by Horuk, R., 1994, Trends Pharmacol. Sci, 15:159-165; Murphy, P. M., 1994, Annu. Rev. Immunol., 12:593-633). The amino terminus of the .beta. chemokines RANTES, MCP-1, and MCP-3 have been implicated in the mediation of cell migration and inflammation induced by these chemokines. This involvement is suggested by the observation that the deletion of the amino terminal 8 residues of MCP-1, amino terminal 9 residues of MCP-3, and amino terminal 8 residues of RANTES and the addition of a methionine to the amino terminus of RANTES, antagonize the chemotaxis, calcium mobilization and/or enzyme release stimulated by their native counterparts (Gong et al., 1996 J. Biol. Chem. 271:10521-10527; Proudfoot et al., 1996 J. Biol. Chem. 271:2599-2603). Additionally, .alpha. chemokine-like chemotactic activity has been introduced into MCP-1 via a double mutation of Tyr 28 and Arg 30 to leucine and valine, respectively, indicating that internal regions of this protein also play a role in regulating chemotactic activity (Beall et al., 1992, J. Biol. Chem. 267:3455-3459).
The monomeric forms of all chemokines characterized thus far share significant structural homology, although the quaternary structures of .alpha. and .beta. groups are distinct. While the monomeric structures of the .beta. and .alpha. chemokines are very similar, the dimeric structures of the two groups are completely different. An additional chemokine, lymphotactin, which has only one N terminal cysteine has also been identified and may represent an additional subgroup (.gamma.) of chemokines (Yoshida et al., 1995, FEBS Lett. 360:155-159; and Kelner et al., 1994, Science 266:1395-1399).
Receptors for chemokines belong to the large family of G-protein coupled, 7 transmembrane domain receptors (GCR's) (See, reviews by Horuk, R., 1994, Trends Pharmacol. Sci. 15:159-165; and Murphy, P. M., 1994, Annu. Rev. Immunol. 12:593-633). Competition binding and cross-desensitization studies have shown that chemokine receptors exhibit considerable promiscuity in ligand binding. Examples demonstrating the promiscuity among .beta. chemokine receptors include: CC CKR-1, which binds RANTES and MIP-1.alpha. (Neote et al., 1993, Cell 72: 415-425), CC CKR-4, which binds RANTES, MIP-1.alpha., and MCP-1 (Power et al., 1995, J. Biol. Chem. 270:19495-19500), and CC CKR-5, which binds RANTES, MIP-1.alpha., and MIP-1.beta. (Alkhatib et al., 1996, Science, in press and Dragic et al., 1996, Nature 381:667-674). Erythrocytes possess a receptor (known as the Duffy antigen) which binds both .alpha. and .beta. chemokines (Horuk et al., 1994, J. Biol. Chem. 269:17730-17733; Neote et al., 1994, Blood 84:44-52; and Neote et al., 1993, J. Biol. Chem. 268:12247-12249). Thus the sequence and structural homologies evident among chemokines and their receptors allows some overlap in receptor-ligand interactions.
CC CKR-5 is the major coreceptor for macrophage-tropic strains of HIV-1 (Alkhatib et al., 1996, Science, in press; Choe et al., 1996, Cell 85:1135-1148; Deng et al., 1996, Nature 381:661-666; Doranz et al., 1996, Cell 85:1149-1158; Dragic et al., 1996, Nature 381:667-674). RANTES, MIP-1.alpha., or MIP-1.beta., the chemokine ligands for this receptor have been shown to block HIV Env-mediated cell fusion directed by CC CKR-5 (Alkhatib et al., 1996, Science, in press; and Dragic et al., 1996, Nature 381:667-674). Additional support for the role of CC CKR-5 as an M-tropic HIV-1 cofactor comes from the finding that a 32-base pair deletion in the CC CKR-5 gene found in three multiply exposed but uninfected individuals, prevents HIV from infecting macrophages (Liu et al., 1996, Cell 86:367-377). However, only three of the 25 uninfected individuals studied had this mutation.
The V3 loop of gp120 is the major determinant of sensitivity to chemokine inhibition of infection or replication (Cocchi et al., 1996, Nature Medicine 2:1244-1247; and Oravecz et al., 1996, J. Immunol. 157:1329-1332). Signal transduction through .beta. chemokine receptors is not required for inhibition of HIV infection or replication, since RANTES inhibits HIV-1 infection in the presence of pertussis toxin, an inhibitor of G-protein-mediated signaling pathways (P. M. Murphy 1994, Ann. Rev. Immunol. 12:593-633; Bischoff et al., 1993, Eur. J. Immunol. 23:761-767; and Simon et al., 1991, Science 252:802-807). CxC CKR4, a CxC (.alpha.) chemokine receptor, has been shown to be a coreceptor involved in infection by laboratory-adapted HIV-1 strains (Fong et al., 1996, Science 272:872-877). The .alpha. chemokine SDF-1, the ligand for this receptor, has been demonstrated to block infection by T-tropic HIV-1 isolates. CxC CKR4 does not bind the beta chemokines RANTES, MIP-1.alpha., or MIP-1.beta..
Recently, it has been shown that certain primary, syncytium-inducing/T-tropic isolates use both CC CKR5 and CxC CKR4 as coreceptors and are able to switch between the two. Thus, in the presence of RANTES, MIP-1.alpha. and MIP-1.beta., the chemokine ligands for CC CKR5, T-tropic strains are still able to infect cells via the CxC CKR4 coreceptor (Zhang et al., 1996, Nature 383:768).
HIV infection is pandemic and HIV-associated diseases represent a major world health problem. Although considerable effort is being put into the design of effective therapeutics, currently no curative anti-retroviral drugs against AIDS exist. In attempts to develop such drugs, several stages of the HIV life cycle have been considered aLs targets for therapeutic intervention (Mitsuya et al., 1991, FASEB J. 5:2369-2381). Many viral targets for intervention with the HIV life cycle have been suggested, as the prevailing view is that interference with a host cell protein would have deleterious side effects. For example, virally encoded reverse transcriptase has been one focus of drug development. A number of reverse-transcriptase-targeted drugs, including 2',3'-dideoxynucleoside analogs such as AZT, ddI, ddc, and d4T have been developed which have been shown to been active against HIV (Mitsuya et al., 1991, Science 249:1533-1544).
The new treatment regimens for HIV-1 show that a combination of anti-HIV compounds, which target reverse transcriptase (RT), such as azidothymidine (AZT), lamivudine (3TC), dideoxyinosine (ddi), dideoxycytidine (ddc) used in combination with an HIV-1 protease inhibitor have a far greater effect (2 to 3 logs reduction) on viral load compared to AZT alone (about 1 log reduction). For example, impressive results have recently been obtained with a combination of AZT, ddI, 3TC and ritonavir (Perelson et al., 1996, Science 15:1582-1586). However, it is likely that long-term use of combinations of these chemicals will lead to toxicity, especially to the bone marrow. Long-term cytotoxic therapy may also lead to suppression of CD8.sup.+ T cells, which are essential to the control of HIV, via killer cell activity (Blazevic et al., 1995, AIDS Res. Hum. Retroviruses 11:1335-1342) and by the release of factors which inhibit HIV infection or replication, notably the chemokines Rantes, MIP-1.alpha. and MIP-1.beta. (Cocchi et al., 1995, Science 270:1811-1815). Another major concern in long-term chemical anti-retroviral therapy is the development of HIV mutations with partial or complete resistance (Lange, J. M., 1995, AIDS Res. Hum. Retroviruses 10:S77-82). It is thought that such mutations may be an inevitable consequence of anti-viral therapy. The pattern of disappearance of wild-type virus and appearance of mutant virus due to treatment, combined with coincidental decline in CD4.sup.+ T cell numbers strongly suggests that, at least with some compounds, the appearance of viral mutants is a major underlying factor in the failure of AIDS therapy.
Attempts are also being made to develop drugs which can inhibit viral entry into the cell, the earliest stage of HIV infection, by focusing on CD4, the cell surface receptor for HIV. Recombinant soluble CD4, for example, has been shown to inhibit infection of CD4.sup.+ T cells by some HIV-1 strains (Smith et al., 1987, Science 238:1704-1707). Certain primary HIV-1 isolates, however, are relatively less sensitive to inhibition by recombinant CD4 (Daar et al., 1990, Proc. Natl. Acad. Sci. USA 87:6574-6579). In addition, recombinant soluble CD4 clinical trials have produced inconclusive results (Schooley et al., 1990, Ann. Int. Med. 112:247-253; Kahn et al., 1990, Ann. Int. Med. 112:254-261; Yarchoan et al., 1989, Proc. Vth Int. Conf. on AIDS, p. 564, MCP 137).
The late stages of HIV replication, which involve crucial virus-specific processing of certain viral encoded proteins, have also been suggested as possible anti-HIV drug targets. Late stage processing is dependent on the activity of a viral protease, and drugs are being developed which inhibit this protease (Erickson, J., 1990, Science 249:527-533). The clinical outcome of these candidate drugs is still in question.
Attention is also being given to the development of vaccines for the treatment of HIV infection. The HIV-1 envelope proteins (gp160, gp1120, gp41) have been shown to be the major antigens for anti-HIV antibodies present in AIDS patients (Barin et al., 1985, Science 228:1094-1096). Thus far, therefore, these proteins seem to be the most promising candidates to act as antigens for anti-HIV vaccine development. Several groups have begun to use various portions of gp160, gp120, and/or gp41 as immunogenic targets for the host immune system. See for example, Ivanoff et al., U.S. Pat. No. 5,141,867; Saith et al., WO 92/22654; Shafferman, A., WO 91/09872; Formoso et al., WO 90/07119. To this end, vaccines directed against HIV proteins are problematic in that the virus mutates rapidly rendering many of these vaccines ineffective. Clinical results concerning these candidate vaccines, however, still remain far in the future.
Thus, although a great deal of effort is being directed to the design and testing of anti-retroviral drugs, effective, non-toxic treatments are still needed.
Citation of a reference hereinabove shall not be construed as an admission that such reference is prior art to the present invention.